i need a new gas tank for 68 firebird....need advice on buying the replacement tank or an in the trunk tank...........can you get a tank with a drain plug so you can drain it before or  after prolonged  storage.........what are the pros and cons ??  

Year One and others have the new fuel tank for your 68, none of the factory replacements have a drain plug and using a fuel cell in the trunk will leave the car very non-original (not to mention the venting and plumbing issues). How long is "prolonged" storage? If it's over the winter or a year, I would recommend keeping the tank full and adding Stabil or another fuel stabilizer according to the directions on the bottle to keep it fresh. Don't forget to add the Stabil first then drive the car a bit to mix it and get the treated gas into the lines and carb, if you don't, the gas in the lines and carb can go stale and cause troubles even though the fuel in the tank is good.  

If you buy a new tankI would suggest considering adding a drain if you want one . If you do so with a new tank there are no BIG BOOM issues. The easiest way may be to look at your current tank with the car on level surface and decide the lowest point at the front edge and mark the new tank in the same spot. Then take it and have a bung or half coupling welded in and add a plug screwed in. A wedge shaped sump like those from SUMMIT may be even better.They come with threaded ports for elect pumps and can be plugged .
 I would agree that a fuel cell may not be the way to go. "Been there dealt with that" .For safety you would need a solid bulkhead added to rear seat area. Then you would need to open trunk to fill . A lot of BS for a non race ride.
 This will sound silly but ask a PRO small engine repair guy  his opinion on BRIGGS/STRATTON stabilizer for fuel. If you think about it those engines sit the longest with no use on a regular basis.

                   Good Luck Eddie